Chapter 1: GIS Data Outline Representing the world as a map Coordinate systems Map scale Data quality issues About ArcGIS

Representing the World as a map Annotation features Point features Line features Polygon features

Scale Scale is the ratio of size on the ground to size on the map Scale influences the generalization of features 1:10,000

Polygons at one scale may be points or lines at a different scale Large Scale map Small Scale map

Data Model 1. Vector Model Features (Spatial objects) are stored as a series of x-y coordinates in a rectangular coordinate system. Features can have one of three geometry types: points, lines, or polygons. Vertic es Polygon Point s Node s Line X Y

Features and attributes Each feature is linked to an entry in a data table containing information about the feature—its attributes.

Feature Classes A feature class is a collection of similar objects with the same attributes, stored as a single unit. Stored as spatial features with a table of associated attributes for each feature. Because all features must share the same table, all must have the same attributes. Feature classes may contain only one type of geometry (points or lines or polygons). Cities feature class Rivers feature class States feature class

Advantages of vectors Precise location of features Storing many attributes Flexible for cartography Compact storage of information Ideally suited for certain types of analysis, especially areas, lengths, connections

2. Raster Model

Continuous Data Raster is the best way to store continuously changing values such as elevation or distance Analysis faster and more flexible than vectors for many applications Some analysis only possible using rasters

Discrete rasters Discrete rasters essentially store features—but in raster format Have relatively few values that change abruptly from one category to another

Continuous rasters Continuous rasters store surfaces or fields of variables that change continuously over space Many potential values. Adjacent cells rarely share the same value.

Coordinate Systems Every GIS data set uses x, y, and sometimes z values to locate geographic data The choice of values is the coordinate system Data set is said to be georeferenced Vertices Polygon Points Nodes Line X Y X, Y location

Coordinate systems Unprojected (GCS) –Geographic coordinate system –Based on spherical coordinates –Degrees of latitude and longitude Projected –Converts spherical coordinates to flat, planar coordinates –Set of mathematical equations –Projects 3D coordinates to 2D map

Map scale Ratio of distance on the map to distance on the ground Scale is dimensionless: could be cm or inches or mm…

A large denominator gives a small fraction  a small scale map. It shows a large area. A small denominator gives a larger fraction  a large scale map. It shows a small area , ,000, ,000

Data Quality Issues Data sets are rarely perfect No absolute quality standard Quality is defined as the fitness of a data set for a particular purpose Same data set may be unsuitable for one use but adequate for another User has ethical and legal responsibility to determine if a data set is sufficient for its intended use

About ArcGIS ArcGIS Desktop ArcToolbox ArcCatalog ArcMap

Storing data in ArcGIS 1.Coverages 2.Shapefiles 3.Gepdatabases

Other types of data

References Price, M. (2013). Mastering ArcGIS (6th ed.). McGraw-Hill Price, M. (2013). Mastering ArcGIS (6th ed.). McGraw-Hill. Mastering ArcGIS, 6/e Instructor Edition Chapter 1: PowerPoint Notes and Figures Instructor Edition Chapter 1: PowerPoint Notes and Figures